A shader is a small program used in computer graphics to determine how surfaces appear in a 3D environment. It controls the color, brightness, and texture of objects by processing light and shadow effects. Shaders are essential for creating realistic images in video games and movies, making them look more lifelike and visually appealing. There are different types of shaders, such as vertex shaders and fragment shaders. The vertex shader processes the shape and position of objects, while the fragment shader handles the final color and texture of each pixel. Together, they help bring digital worlds to life.